To improve the efficiency of solar batteries, it is necessary to suppress recombination of carriers. In most crystalline silicon solar batteries currently commercially available, a high concentration diffusion layer of the same conductive type as that of the substrate of the solar battery is provided on the back surface of the substrate to remove minority carriers on the back surface of the substrate by the built-in potential of the junction, whereby recombination on the back surface of the substrate is suppressed. The high concentration diffusion layer on the back surface of the substrate is referred to as a BSF (Back Surface Filed) layer.
In a typical silicon solar battery, a combination of a p-type substrate and a BSF layer formed by diffusing aluminum (Al) into the back surface of the p-type substrate is used. More specifically, in such a typical silicon solar battery, an aluminum (Al) paste is printed on the back surface of the substrate and is then fired to form back surface electrodes, and aluminum (Al) is diffused into the back surface of the substrate to form a BSF layer (hereinafter denoted as an Al-BSF layer).
A recent increase in market size results in a shortage of silicon materials for solar batteries, and therefore their manufacturers are making efforts to reduce the thicknesses of the solar batteries. However, since the thermal expansion coefficients of silicon (Si) and an Al—Si alloy (an Al-BSF layer) are different, the warpage of a solar battery cell increases as the thickness of the silicon substrate decreases, and this affects the subsequent module production step. Therefore, the use of such an Al-BSF layer is quite inconvenient for thin silicon substrates.
Accordingly, development of back surface passivation technique as an alternative to the Al-BSF layer is in progress. In the field of single crystal silicon solar batteries, back surface passivation technique alternative to the Al-BSF layer has been developed to improve the efficiency of solar batteries, although this technique is still at a research level. PERC (Passivated Emitter and Rear Cell) cells and PERL (Passivated Emitter Rear Locally diffused) cells developed in University of New South Wales, Australia correspond to the above technique.
In these solar batteries, a silicon oxide film (SiO2) is formed on the back surface of the silicon substrate by thermal oxidation to passivate the back surface of the silicon substrate. However, the step of forming the silicon oxide film (SiO2) by thermal oxidation is a high temperature process at 1,000° C. or higher. Therefore, if this step is applied to polycrystalline silicon substrates, which are the mainstream in the current market, the quality of the crystals is impaired significantly, so that thermal oxidation cannot be applied to solar batteries that use polycrystalline silicon substrates.
In view of the above, Schultz et al. have achieved improvement in efficiency of a polycrystalline silicon solar battery by forming a silicon oxide film (SiO2) on the back surface of the silicon substrate by wet oxidation at 800° C. to passivate the back surface of the silicon substrate (see, for example, Non-Patent Document 1). However, the oxidation time in the step of forming the silicon oxide film (SiO2) by wet oxidation is several hours, and therefore the mass productivity of this process is not high.
Therefore, there is a need for a film especially for polycrystalline silicon solar batteries that can be formed by a low-temperature process and has high mass productivity and good passivation characteristics.
In polycrystalline silicon solar batteries, a silicon nitride film formed by PECVD (Plasma Enhanced Chemical Vapor Deposition) (an SiN film, hereinafter denoted as a PECVD-SiN film) is used as a front surface passivation film serving also as an antireflection film. This is because hydrogen contained in the PECVD-SiN film diffuses into grain boundaries during firing of electrodes and defects in the silicon substrate are passivated, so that the effect of improving conversion efficiency is thereby obtained. It is therefore natural to contemplate passivating the back surface of a silicon substrate using a PECVD-SiN film, and various research groups are studying the passivation with such a PECVD-SiN film.